The exact method to transition from Truegold to Underlord can be a bit of a mystery for some sacred artists, and many are never able to cross that line. It requires you to know yourself, and your motivation for following the sacred arts. Once that personal insight is achieved then the sacred artist must reach outward to the vital aura all around them, become infused with soulfire, and be reborn as an Underlord. It is certainly a coveted rank and desired by all Truegold sacred artists, including Lindon, but for Yerin, this step in her progression has literally become a matter of life and death. After sustaining irreparable damage to her lifeline in a battle, Yerin's life has been shortened to the point where her death is imminent, unless she manages to be reborn during the transformation to Underlord before her lifeline runs out. No pressure.
Underlord - the next step in sacred artist progression